The present invention relates to an cutoffless class B emitter-follower type single-ended push-pull (SEPP) circuit.
In general, in an emitter-follower type SEPP circuit, class B amplification is employed for reasons of efficiency. In such a circuit, it is essential to cause an idle current to flow in order to smoothly connect the upper and lower transfer characteristics. In a conventional circuit of this type, when one of the transistors is turned on, the other transistor is cut off, and switching distortion takes place when switching from one transistor to the other. In order to eliminate this difficulty, a cutoffless class B circuit is often employed in which each transistor is prevented from being cut off using a servo circuit. In this case, the switching distortion is largely eliminated. However, it should be noted that there is still present a current distortion due to the inherent nonlinearities in the current transfer characteristics of transistors or a voltage distortion due to the exponential voltage transfer characteristics.
In the case of a bipolar transistor, if no temperature compensation is employed, the idle current may undergo thermal runaway. Heretofore, no servo control has been applied to the idle current. Accordingly, the idle current value changes with variations of signal strength and ambient temperature. Irrespective of the presence or absence of an input signal, the operating point tends to change over both long and short periods.
Temperature compensation is considerably difficult to implement. Especially in the conventional cutoffless class B circuit, a positive feedback technique must be employed, and therefore the instability of the idle current is increased. This, in association with the fact that temperature compensation cannot be effected perfectly, makes the design more difficult.
FIG. 1 shows an example of a conventional cutoffless class B SEPP circuit. Transistors Q.sub.7a and Q.sub.7b, and Q.sub.8a and Q.sub.8b are drive transistors and output transistors which form the SEPP circuit. Idle current stabilizing resistors R.sub.Ea and R.sub.Eb are provided. The SEPP circuit is provided with a voltage amplifying circuit including transistors Q.sub.11a and Q.sub.12a, a resistor R.sub.11a and constant current sources I.sub.11a and I.sub.12a, and also with a voltage amplifying circuit including transistors Q.sub.11b and Q.sub.12b, a resistor R.sub.11b and constant current sources I.sub.11b and I.sub.12b.
When a positive input signal voltage is supplied to the circuit, a current I.sub.E1 flows in the transistor Q.sub.8a so that the voltage between the points p and q is increased. A voltage increment is developed across the resistor R.sub.11a by the transistor Q.sub.12a, which acts as emitter-follower, thus suppressing the variation of the voltage between the points p and r and thus causing an idle current I.sub.d to continuously flow in the transistor Q.sub.8b. Accordingly, when a positive input signal is supplied, the idle current of the transistor Q.sub.8b is not interrupted. Similarly, in the case when a negative input signal voltage is applied, the increment of the voltage between the points p and r is developed across the resistor R.sub.11b by the transistor Q.sub.12b, thus causing the idle current to continuously flow in the transistor Q.sub.8a. Thus, the circuit acts as a cutoffless class B SEPP circuit. In this case, the transfer characteristic is as indicated by the curve a.sub.0 in FIG. 2. In FIG. 2, the curves b.sub.0 and c.sub.0 indicate the transfer characteristics of the transistors Q.sub.7b and Q.sub.8b and of the transistors Q.sub.7a and Q.sub.8a, respectively.
In the conventional class B SEPP circuit, to set the idle current, after the transistors Q.sub.12a and Q.sub.12b are placed substantially in the cutoff state by adjusting a variable resistor V.sub.R2, the idle current is set to a predetermined value by adjusting a variable resistor V.sub.R1. The idle current thus controlled changes with time, temperature and supply voltage, and the operating point also changes. Thus, it is difficult to control the idle current.
Furthermore, it is impossible to accurately predict the value of the idle current when the input signal is supplied.
The circuit should be temperature compensated, but it is impossible to provide 100% temperature compensation.
If the positive feedback ratio is set to unity so that the voltage between the points p and q is reflected across the resistor R.sub.11a, then the stabilizing action of the resistor R.sub.Ea is completely lost, as a result of which there is a greater tendency for oscillation or thermal runaway to occur. Therefore, the positive feedback ratio must be less than unity. Accordingly, when a large signal is inputted, complete noncutoff operation is not obtainable. In order to obtain complete noncutoff when a large signal is present, it is necessary to set the idle current to a large value, which reduces the gain of the circuit.
Furthermore, the output impedance is high and changes with the level of the input signal voltage. Also, the distortion due to the exponential transfer characteristics of the transistors Q.sub.7a, Q.sub.7b, Q.sub.8a and Q.sub.8b is not significantly reduced. The transfer characteristic is nonlinear as indicated by the curve a.sub.0 in FIG. 2, and the composite output current is significantly distorted as shown in FIG. 3. Thus, the conventional cutoffless class B SEPP circuit is disadvantageous in various points.